[Tagging] Using restriction and restriction:vehicle for the same restriction relation should be discouraged

[Minh Nguyen]

Vào lúc 01:07 2022-10-29, Tobias Knerr đã viết:
> On 29.10.22 07:13 easbar.mail at posteo.net 
> wrote:
>> I like your idea of not using the except tag but rather something like 
>> restriction:value=unrestricted. Actually that would be the first 
>> useful combination of restriction and restriction:vehicle that I have 
>> heard of. But unfortunately this is neither mentioned in the wiki nor 
>> does it seem to be used that way.
> 
> Yes, this is something that would require a proposal to introduce, it's 
> not established practice at this point. The reason I mention it in this 
> discussion is indeed that it would be one example where restriction and 
> restriction:vehicle on the same relation could meaningfully coexist. So 
> I wonder if a wording could be used that would leave this door open for 
> the future, e.g. by discouraging "different types of restriction on the 
> same relation" (or some better wording to the same effect).
> 
>> I'm still wondering: Do we ever need different restriction values for 
>> different vehicles, or for different conditions, for the same relation?
> 
> No, with the exception of the "unrestricted" concept I mentioned, this 
> should never be necessary. Even if there is an obscure real-world 
> situation with different restrictions on the same turn (same 
> from-via-to) for different vehicles, it could be modelled with separate 
> relations.

For context, this question came up the other day in the iD bug tracker. 
[1] It was unclear how an editor should depict a mixed-restriction 
relation. If it's unclear for editors, it's certainly unclear for routers.

In a sense, this is actually a common phenomenon: trucks must exit to a 
weigh station but cars may not. Or cyclists must turn right from a bike 
lane to a bike path but cars may not. However, these restrictions are 
already represented by access restrictions on the ways themselves -- no 
need for relations. Normally, way-based restriction tagging breaks down 
when the restriction depends on the direction of approach, but that 
would require separate relations anyhow.

One potential issue with relying on separate relations is that there's 
no way to indicate an order of precedence explicitly. For example, at 
one of San Francisco's most prominent intersections, an "except Muni" 
sign modifies a no left turn sign, exempting one bus operator that comes 
from the west, but not AC Transit. [2] At the same intersection, "no 
right turn except Muni and SamTrans" exempts both bus operators that 
routinely come from the south. [3]

Most of the bus routes in this part of town follow strict paths, so I 
just coined an obscure except:network=* tag and called it a day. [4] But 
this experience leads me to suspect there may be places where the 
real-world restrictions turn our usual access key hierarchy [5] on its 
head. Unfortunately, there's nothing to say that one relation takes 
precedence over another relation with the same members. (Incidentally, 
this is why I've shied away from proposing that we use relations to 
track overlapping parking lane restrictions.)

[1] https://github.com/openstreetmap/iD/issues/9337#issuecomment-1283492156
[2] 
https://commons.wikimedia.org/wiki/File:City_of_San_Francisco_(Unsplash).jpg
[3] https://www.mapillary.com/app/?pKey=4265238293573050
[4] https://www.openstreetmap.org/relation/10650926
[5] https://wiki.openstreetmap.org/wiki/Key:access#Land-based_transportation

-- 
minh at nguyen.cincinnati.oh.us